SW

4.7pF

C3

4.72H

L1

FREQ

7

2

6

4

8

3

1

PGND

SGND

BOOT

EN

FB

D1

3A

5V

RF2

100k

C1

222F

V

IN

2 Cells

R3

150k

RF1

300k

+

V

DD

LM2623

C2

1002F

tant

5

LM2623

SNVS188H – MAY 2004 – REVISED NOVEMBER 2014

www.ti.com

8 Applications and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

The LM2623 features a shutdown mode, entry into the shutdown mode is controlled by the active-low logic input

8.2 Typical Application

Figure 9. LM2623 Typical Application

8.2.1 Design Requirements

The LM2623 allows the designer to vary output voltage, operating frequency and duty cycle to optimize the part

performance, please read Detailed Design Procedure for details.

8.2.2 Detailed Design Procedure

8.2.2.1 Non-Linear Effect

The LM2623 is very similar to the LM2621. The LM2623 is based on the LM2621, except for the fact that the

LM2623 takes advantage of a non-linear effect that allows for the duty cycle to be programmable. The C3

capacitor is used to dump charge on the FREQ pin in order to manipulate the duty cycle of the internal oscillator.

The part is being tricked to behave in a certain manner, in the effort to make this Pulse Frequency Modulated

(PFM) boost switching regulator behave as a Pulse Width Modulated (PWM) boost switching regulator.

8.2.2.2 Choosing the Correct C3 Capacitor

The C3 capacitor allows for the duty cycle of the internal oscillator to be programmable. Choosing the correct C3

capacitor to get the appropriate duty cycle for a particular application circuit is a trial and error process. The non-

linear effect that C3 produces is dependent on the input voltage and output voltage values. The correct C3

capacitor for particular input and output voltage values cannot be calculated. Choosing the correct C3

capacitance is best done by trial and error, in conjunction with the checking of the inductor peak current to make

sure your not too close to the current limit of the device. As the C3 capacitor value increases, so does the duty

cycle. And conversely as the C3 capacitor value decreases, the duty cycle decreases. An incorrect choice of the

C3 capacitor can result in the part prematurely tripping the current limit and/or double pulsing, which could lead

to the output voltage not being stable.

10

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